1. Field of the Invention
The present invention relates to an interface circuit, and more particularly to an interface circuit coupling a semiconductor test apparatus with a tested semiconductor device.
2. Description of the Background Art
Conventionally, in the field of semiconductor integrated circuit devices (referred to as LSI hereinafter), a test is performed before shipment to determine whether or not each LSI is normal and only a normal LSI is shipped. In this test, a plurality of LSIs are connected to one semiconductor test apparatus (referred to as a tester hereinafter). Usually, one external terminal of LSI is connected to one external pin of the tester, and a signal is applied, for example, from the external pin of the tester to the external terminal of LSI.
In order to reduce the test cost for LSI, a method of connecting an output pin of a tester to a plurality of LSIs in parallel has been proposed (for example, see Japanese Patent Laying-Open No. 2002-189058).
With only parallel connection of an output pin of a tester with a plurality of LSIs, a mismatch of an output impedance of the tester may be caused to deteriorate a waveform quality of the output signal or an output current of the tester may not be distributed evenly to a plurality of LSIs, thereby preventing an accurate test.
Recently, with the advancement of the process technology, low power supply voltage type of LSIs are increased in number in addition to conventional high power supply voltage type of LSIs. However, the low power supply voltage type of LSI cannot be tested with the tester that has tested the high power supply voltage type of LSI, because the resolution of the output voltage is coarse. Therefore, a tester having a high voltage-accuracy is separately needed, thereby increasing the test cost.
In addition, with the lower consumption of LSI, the output current of LSI is reduced and the output impedance of LSI is increased. Therefore, because of a mismatch between an impedance of an external pin of a commercially available tester (mainly 50 Ω) and an output impedance of LSI (100-300 Ω), the output signal waveform of LSI suffers from the effect of reflection. This effect prevents an accurate measurement by the tester.